Combustion engines such as diesel engines, gasoline engines, and gaseous fuel-powered engines are supplied with a mixture of air and fuel for combustion within the engine that generates a mechanical power output. In order to maximize the power output generated by this combustion process, the engine is often equipped with a divided exhaust manifold in fluid communication with a turbocharged air induction system.
The divided exhaust manifold increases engine power by helping to preserve exhaust pulse energy generated by the engine's combustion chambers. Preserving the exhaust pulse energy improves turbocharger operation, which results in a more efficient use of fuel. In addition, the turbocharged air induction system increases engine power by forcing more air into the combustion chambers than would otherwise be possible. This increased amount of air allows for enhanced fueling that further increases the power output generated by the engine.
In addition to the goal of maximizing engine power output and efficiency, it is desirable to simultaneously minimize exhaust emissions. That is, combustion engines exhaust a complex mixture of air pollutants as byproducts of the combustion process. And, due to increased attention on the environment, exhaust emission standards have become more stringent. The amount of pollutants emitted to the atmosphere from an engine can be regulated depending on the type of engine, size of engine, and/or class of engine.
One method that has been implemented by engine manufacturers to comply with the regulation of these exhaust emissions includes utilizing an exhaust gas recirculating (EGR) system. EGR systems operate by recirculating a portion of the exhaust produced by the engine back to the intake of the engine to mix with fresh combustion air. The resulting mixture has a lower combustion temperature and, subsequently, produces a reduced amount of regulated pollutants.
EGR systems require a certain level of backpressure in the exhaust system to push a desired amount of exhaust back to the intake of the engine. And, the backpressure needed for adequate operation of the EGR system varies with engine load. Although effective, utilizing exhaust backpressure to drive EGR can adversely affect engine operation, thereby reducing fuel economy. Thus, a system is required that reduces exhaust backpressure, while still providing the necessary EGR flow.
U.S. Pat. No. 6,324,847 to Pierpont (“the '847 patent”) discloses a combustion engine utilizing an EGR system and a divided exhaust manifold together with a turbocharged air induction system. Specifically, the '847 patent describes an internal combustion engine suitable for a vehicle and having a plurality of combustion cylinders. A first exhaust manifold and a second exhaust manifold are separately coupled with the plurality of combustion cylinders. The EGR system is disposed in fluid flow communication between the first exhaust manifold and an intake manifold of the engine. A turbocharger associated with the engine has a compressor with an inlet receiving combustion gas and an outlet in fluid flow communication with the intake manifold, and a turbine drivingly coupled to the compressor and having a turbine casing and a turbine wheel. The turbine casing defines separate first and second inlet paths for exhaust gas flow from the first and second exhaust manifolds. The engine is also provided with a valve for controlling the flow of exhaust gases through the EGR system, and a cooler located downstream of the valve to cool the exhaust gases. Efficiency and performance are improved by preserving pulse energy in one exhaust manifold flow, and increasing back pressure in the manifold providing EGR flow.
Although the system in the '847 patent may adequately control exhaust gas recirculation in a turbocharged engine, it may be less than optimal. Specifically, because the '847 patent provides only single-stage turbocharging, it may be applicable to only low-boost applications.
The disclosed exhaust system is directed to overcoming one or more of the problems set forth above and/or other problems of the prior art.